Compression release for an internal combustion engine

Abstract: A compression release system is provided in an engine having a main exhaust port (48) with an attached muffler (28). A muffler supporting boss (40) extends from the cylinder of the engine separate from the muffler mounting at the main exhaust port. A compression release port (44) extends through the boss (42). The muffler has two separate inlet apertures (56,58); one for the main exhaust port (56) and one for the compression release port (58).

Description: The present invention relates to internal combustion engines and, more particularly, to a compression release system. U.S. Patent 4,252,092 discloses a compression release mechanism where gases from a compression release port are transported to a muffler of the engine. U.S. Patent 3,538,899 discloses a similar mechanism with a valve located in the compression relief port. Other U.S. patents that relate to compression release include: 4,619,228; 2,742,380; 5,054,441; 4,993,372; 4,312,308; 3,417,740; 2,023,048; 1,377,139; 4,791,892 and 5,134,976. A muffler with two inlet apertures is also known in the prior art with one aperture connected to the main exhaust port of a two stroke engine and the other aperture connected to a port at the crankcase area of the engine. Other documents relating to engines usable in power tools, lawn mowers and like purposes include GB 1,529,925; FR 2,525,687, Japanese Publication 1066415, and US Patent 5,127,372. Japanese Published Patent Application No. 64-87826 of Suzuki Motor Co. Limited discloses an i.c. engine comprising a pair of cylinders 1a and 1b. Each cylinder has a main exhaust port (8a or 8b) and small port (16a,16b) which also allows for release of gases. The ports open into a common zone 9 as seen in Figure 2 of the Japanese document. In accordance with the present invention, there is provided an internal combustion engine having a compression release system and comprising all features of appended claim 1. In accordance with an embodiment of the present invention, a power driven tool having an internal combustion engine is provided, the engine comprising a cylinder and a muffler. The cylinder has a muffler mounting section. The muffler mounting section includes a main exhaust port extending through a first portion and a compression release port extending through a second portion. The first and second portions are spaced from each other. The muffler is connected to the cylinder on the first and second portions and includes a first inlet aperture connected to the main exhaust port and a second inlet aperture connected to the compression release port. The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings, wherein: Fig. 1 is a perspective view of a string trimmer having an engine incorporating features of the present invention. Fig. 2 is a schematic cross sectional view of the cylinder, piston, and muffler of the engine shown in Fig. 1. Fig. 3 is an enlarged sectional view of the compression release port shown in the cylinder of Fig. 2. Fig. 4 is a partial schematic sectional view of an alternate embodiment of the present invention. Referring to Fig. 1 there is shown a perspective view of a string trimmer 10 incorporating features of the present invention. Although the present invention will be described with reference to the embodiments shown in the drawings, it should be understood that the present invention can be embodied in various different types and kinds of alternate embodiments. The present invention may also be embodied in different types of power driven tools including, but not limited to, chain saws, hedge trimmers, fluid pumps, leaf blowers, demolition saws, lawn mowers, or any other type of tool or machine driven by an internal combustion engine. In addition, any suitable size, shape, or type of elements or materials could be used. The string trimmer 10 includes a cutting head 12, a shaft 14, a front handle 16, a rear handle 18 with throttle trigger 20, and an internal combustion engine 22. String trimmers are well known in the art. The only difference between the string trimmer 10 and known string trimmers is in the engine 22. Therefore, the known conventional features of the string trimmer will not be described in any further detail. Referring also to Figs. 2 and 3, the engine 22 is a two stroke engine with a cylinder 24, a piston 26, and a muffler 28. The engine 10 obviously has features such as a gas tank, carburetor, air filter, etc. which are conventional and need not be further described. The cylinder 24 has a piston cylinder space 30 in which the piston 26 reciprocally moves. The cylinder 24, in the embodiment shown, includes an air and fuel inlet port (not shown), a main exhaust port 32, and a compression release port 34. A spark plug 36 is connected to the cylinder 24 at a top end 38 of the cylinder 24. In the embodiment shown, the cylinder 24 is a one piece member, but in alternate embodiments it may be comprised of multiple members connected to each other. The cylinder 24 has a muffler mounting section that includes a first portion 40 and a second portion 42. The first portion 40 has the main exhaust port 32 extending therethrough. The first and second portions 40, 42 are separated or spaced from each other along the length of the cylinder. In the embodiment shown, the first and second portions are integrally formed with the cylinder. However, in alternate embodiments, the first and/or second portions could include a member or members that are attached to the cylinder. The entrance 44 to the compression release port 34 is located about half-way between the top 46 of the entrance 48 to the main exhaust port 32 and the upper most position of the top 50 of the piston 26 (also known as "top dead center"), illustrated by line A, as the piston reciprocates. The compression release port 34 generally comprises a first small section 52 and a second enlarged section 54. In a preferred embodiment the length B of the first small section is smaller than the diameter C of the compression release port in the first small section 52. One example would comprise the diameter C as being 2mm (0.080 inch) and the wall thickness or length B being 1mm (0.040 inch). However, other sizes and proportions could be provided in alternate' embodiments. The second enlarged section 54 is preferably at least about 30 percent larger in diameter D than the diameter C. In the example given above, the diameter D would be about 3.2 mm (0.125 inch). Although diameters are used above to describe the cross-sectional areas of sections 52 and 54, it should be understood that cross-sectional shapes could be provided other than circular. The muffler 28 is of conventional design with one major exception. Instead of having a single inlet aperture for the main exhaust port, the muffler 28 has two inlet apertures; a first inlet aperture 56 and a second inlet aperture 58. The muffler 28 is fixedly attached to the cylinder 24 at the first portion 40 of the muffler mounting section by use of suitable fasteners (not shown) such as screws. A gasket 60 is located between the muffler 28 and the cylinder 24. The main exhaust port 32 communicates to the first inlet aperture 56. The second portion 42 of the muffler mounting section is provided as a boss that the muffler and gasket press against. The compression release port 34 communicates to the second inlet aperture 58. In an alternate embodiment the muffler could alternatively or additionally be directly fixedly connected to second portion 42 by additional fasteners. In addition, any suitable means to connect the muffler to the cylinder could be used. During starting of the engine 22 a portion of the compression of air and fuel in the area 30 is intentionally lost through the port 34 as the piston 26 moves upward towards the spark plug 36. This intentional loss of compression makes the engine easier to start with a pull starter or electric starter. As the piston 26 moves up, it eventually passes the entrance 44 to the port 34 thereby closing the release port 34. This occurs when the top 50 of the piston 26 passes the point E, about halfway between point A and the top 46 of the main exhaust port entrance 48. If the port 34 was not present, the compression would be about twice what it is for the engine 22. Thus, the actual compression for the engine 22 is about half of the compression of what the engine would have had without the port 34 being present. When the engine starts, the piston 26 is driven down from point A opening the release port 34 when the top 50 reaches point E. Some flow from the power pulse escapes through the release port 34. This escape causes a slight power loss, but the loss is substantially small because the most useful part of the power pulse has already been completed before this point. The flow rate allowed by the port 34, which is sufficient for the low pressure/low velocity escape during compression, is not enough to allow substantial loss of power at the high pressure/high velocity flow from a live power pulse. This feature of flow rate control is accomplished by two factors; the size of the hole 34 relative to the size of the area 30 and its location along the length of the cylinder area 30. The present invention allows much easier starting of the engine with no significant loss in power and, also without significant increase in emissions. It has been found that the size of the compression release hole 34 should be designed to be proportionally sized relative to the engine displacement. Preferably, the ratio of engine displacement (i.e. cylinder cross-sectional area) to the cross-sectional area of the compression release port at small section 52 should be about 750:1. With this ratio, power loss is insignificant, emissions are kept low, and starting of the engine is nonetheless much easier. It has been found that an engine with a ratio of about 300:1 can have significant power loss. Although a preferred ratio of about 750:1 has been described above, other suitable ratios, such as ranging from about 850:1 to about 650:1, could be used. Multiple compression release ports could also be used including staggered along the length of the cylinder 24. Multiple compression release ports could also have different sizes. Another feature of the present invention is the inherent resistance to clogging of the release port 34 and ease of cleaning of the release port 34 if it does become clogged. During use of prior known engines with compression release ports, carbon eventually builds up in the release ports and blocks the ports. To repair the blocked release port, a tool such as a drill or pin has to be used to open the release port. The present invention uses the relatively small length B and enlarged diameter D to help prevent carbon from blocking the port 34. However, even with the improved design described above, carbon may nonetheless build up in port 34 over a long period of time thereby blocking the port. With the preferred embodiment of the present invention, by merely removing the muffler 28, easy access is allowed to the release port 34 for cleaning and/or use of a properly sized pin to push out the carbon. Another feature of this system is the low cost to manufacture and the ease to add to an existing cylinder design. An existing cylinder can be modified by adding a boss to the casting, drilling a through-hole for the first small section of the port, then end milling to a depth for the second enlarged section of the port. The only other modifications necessary are an additional hole in the muffler and muffler gasket for the port exhaust. Fig. 4 shows an alternate embodiment. In the embodiment shown, the cylinder 24a has multiple compression release ports 34a, 34b. The ports have different sizes and are spaced along the length of the cylinder 24a. In this embodiment, the ports are closed and opened sequentially as the piston moved up and down. This type of embodiment can be used for larger size engines. The ports could also be located at the same location of the cylinder, merely being angularly spaced from each other. The ports could also have the same size. The present invention could also be used in four stroke engines. It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.

Claims: An internal combustion engine for a power driven tool, the engine having a compression release system and being characterized in that it comprises: a cylinder (24) having a muffler mounting section, the muffler mounting section having a main exhaust port (32) extending through a first boss (40) on the cylinder (24) and a separate compression release port (34) extending through a second boss (42) on the cylinder (24), the first and second bosses (40,42) being spaced from each other; and a muffler (28) connected to the cylinder (24) on the first and second bosses, the muffler having a first inlet aperture (56) connected to the main exhaust port (32) and a separate second inlet aperture (58) connected to the compression release port (34). An internal combustion engine as claimed in claim 1, in which the cylinder has a main exhaust port (32), a first compression release port (34a), and a second compression release port (34b), and in which all of these ports are separate and spaced from each other, and extend through a wall of the cylinder (24) into a single piston cylinder space. An internal combustion engine as claimed in claim 1, wherein the compression release port is located about half-way between the top of an entrance to the main exhaust port and an upper-most position of the top of a piston of the engine. An internal combustion engine as claimed in claim 1 or 3, wherein the compression release port has a first small section (52) at an entrance to the compression release port and a second enlarged section (34) between the first small section and the second inlet aperture of the muffler. An internal combustion engine as claimed in claim 4 wherein the second enlarged section has a diameter about 30 percent larger than the diameter of the first small section. An internal combustion engine as claimed in claim 4 or 5 wherein the length of the first small section is smaller than a diameter of the compression release port (34). An internal combustion engine as claimed in claim 4, 5 or 6 wherein the length of the first small section is about half the size of the diameter of the compression release port (34). An internal combustion engine as claimed in any preceding claim wherein the compression release port (34) is a straight channel through the second boss (42) perpendicular to the axis of the cylinder (24) and generally parallel to the main exhaust port (32) in the first boss (40). An internal combustion engine as claimed in any preceding claim, wherein the displacement cross-sectional area of the engine and the cross-sectional area of the compression release port are in a ratio of between about 850:1 and about 650:1. An internal combustion engine as claimed in claim 9 wherein the said ratio is about 750:1. An internal combustion engine as claimed in claim 9 which comprises multiple compression release ports for the cylinder. An internal combustion engine as claimed in claim 2 wherein the ports (32, 34a, 34b) are spaced from each other along the length of the cylinder (24).

Inventors: CAMPBELL JACK E, MORROW JOHN A, CAMPBELL, JACK E., MORROW, JOHN A.
Applicants: TEXTRON INC, TEXTRON INC.
Application year: 1994
Main classification: F01L13
IPC codes: F02D15, F02B75, F01N7, F02N17, F01N1, F02B63, F02F1, F01L13